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Abstract

The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-ray-induced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs.

, Electron bunch arrival time data for a typical run. (a) Histogram of measured BATs for both phase cavities. (b) Histogram of BATs for phase cavity two showing the BATs in more detail and a standard deviation of ~112 fs. (c) Time series plots of both phase cavities for successive shots while the machine was running at 30 Hz. The blue trace shows the measured BAT times for phase cavity one while the green trace shows the measured BAT times for phase cavity two (displaced by 0.75 ps for clarity). The dashed lines denote one second intervals. We see that over these short time scales the standard deviation of either cavity is ~100 fs.

, LCLS near experimental hall laser timing system. A signal derived from the average electron beam time of arrival measured in the undulator hall is transmitted through a stabilized fiber system and used to synchronize the optical laser oscillator in the near-experimental hall that is hundreds of feet away. The oscillator pulses are then amplified, converted to beam rate, and transported to the experiment and timing jitter from these processes are not compensated by the timing system.

, Experimental setup showing nitrogen molecules aligned along the iToF axis. The 800 nm alignment laser impulsively aligns molecules along its polarization. X-rays from the LCLS then dissociate these molecules. Nitrogen ions that are emitted towards the repeller plate are directed back towards the iToF, resulting in a time delay difference between these ions and those that are emitted towards the iToF initially.

, The ion ToF spectrum. (a) The N2++ molecules appear as a sharp central feature in the m/q = 14 peak in this spectrum while the dissociated atomic ions are displaced by the momentum imparted by coulomb explosion. When the optical laser pulse succeeds the x-ray pulse (positive delays), the N2++ fraction is decreased by strong-field induced dissociation of the dication. Transient molecular alignment along and perpendicular to the spectrometer axis leads to alternating enhancement in the peak wings and peak center. (b) The black curve is the trace of the ratio of the integrated N+/N2++ peak wings to the integrated central portion showing detailed molecular alignment structure near the molecular half revival. The red curve is a time re-binned data set using ~50 fs time bins showing improvement in the signal fidelity. (c) N2++ center peak intensity vs. pump-probe time delay (black) after subtraction of atomic N+ signal contributions (see text). Strong-field dissociation of N2++ by optical pulses that succeed the x-ray pulse allows for a direct determination of the apparatus function and time zero by a corresponding drop in the N2++ intensity. The blue curve is a Gaussian error function fit to the data with a FWHM of 283 fs that is expected to be dominated by the timing jitter between the laser and the x-rays.